1. Field of the Invention
This invention relates generally to busses for memory cards to allow a Digital Signal Processor (DSP) on a memory DIMM or SIMM to communicate with a system memory controller, and more particularly to the use of a serial bus for communication of a DSP on a memory card with a system memory controller.
2. Background Information
The use of DSPs on memory cards is now being proposed as a way for providing a relatively inexpensive processor on a memory card which can perform tasks on the card while the system bus is otherwise occupied. This works well in so far as it goes; i.e. as long as the DSP needs to be used only on the card on which it resides and does not need to communicate with the system bus this arrangement is fine. However with the present proposals there is no mechanism for the DSP on the card to initiate communication with the system bus and ultimately with devices controlled by the system bus. Since the DSP is a relative powerful processor it is desired that a technique be provided for a DSP to initiate communication effectively and simply with the system bus.
According to the present invention a serial bus and connection to a device on a computer system, preferably through a controller such as a system memory controller is provided on a memory card having a DSP and a memory bus controller to allow the DSP on the memory card to gain access to the device on the system bus without the need to use the system memory bus. The serial bus in its preferred form is a two wire serial bus which connects the device to the DSP through a memory bus controller and preferably through the system memory controller. If more than one memory card is present with DSPs or more than one device contending for access, the system memory controller or other controller will arbitrate the access of each memory card or contending device. In such case the serial bus will signal the system memory controller when it wants access to the particular device, and the system memory controller will act as arbitrator to grant or not grant access to the particular memory card or device requesting access. If access is granted the bus memory controller outputs the required control or command word on the serial bus followed by the address and the required data. This serial information is received by the system memory controller or some other controller which packets it, and, when the transmission is complete, outputs the information rapidly on a parallel bus, e.g. a PCI bus to the device which needs the information. Thus each DSP in effect becomes a bus master when so required for transferring data to and from a device other than the specific memory card on which it resides. The DSP can also take control for reading data from a device using the serial bus.